1. Field of the Invention
The present invention relates generally to a pneumatic actuator for use in a vibration damping device wherein displacement of the elastic wall member of a hermetically sealed working air chamber in association with changes in air pressure therein is converted into output, as well as to a fluid filled type vibration damping device employing the same. More specifically, the present invention is concerned with a pneumatic actuator of novel structure adapted to prevent noise associated with the elastic wall member striking against and moving away from the base housing, and a fluid filled type vibration damping device employing the same.
2. Description of the Related Art
Known pneumatic actuators are typically of designs whereby an output portion undergoes displacement through an externally-induced change in pressure within a working air chamber, and this displacement of the output portion is converted into output. For instance, such an actuator may be utilized as means for switching the vibration damping characteristics in a fluid filled type vibration damping device of pneumatic switching type whose vibration damping characteristics are switchable from the outside. In one design taught for example in JP2003-4090-A1, the pneumatic actuator includes: a base housing; an elastic wall member; a working air chamber hermetically sealed from the outside and formed between opposed faces of the housing and the elastic wall member; and an output portion disposed on the elastic wall member, which is urged in the direction away from the base housing by urging means provided to the working air chamber. Through negative pressure acting on the working air chamber from outside, the output portion will undergo displacement in the direction closer towards the base housing in opposition to the urging force of the urging means. Through atmospheric or positive pressure acting on the working air chamber from outside, the output portion will undergo displacement in the direction away from the base housing under the action of the urging force of the urging means.
In the pneumatic actuator of conventional structure disclosed in JP2003-4090-A1, portions of the opposing face of the base housing and the elastic wall member are parallel to each other, and when negative pressure acts on the working air chamber these opposing parallel faces will come into contact with each other, resulting in partial intimate contact between the base housing and the elastic wall member. Where these contacting faces are parallel faces which have been driven into intimate contact with each other in this way, there is a risk that noise will be produced in association with their release from the state of intimate contact when the pressure in the working air chamber changes from negative pressure to atmospheric pressure.
In particular, where the air pressure in the working air chamber is changed from the outside through a port which has been provided in the diametrical center section, when atmospheric pressure acts on the working air chamber through the port the base housing and the elastic wall member will be released from their state of contact under conditions such that a zone of negative pressure continues to be maintained towards the outer circumferential side from the location of intimate contact between the base housing and the elastic wall member within the working air chamber. Consequently, when the pressure in the working air chamber changes from negative pressure to atmospheric pressure, suctioning action similar to a suction disk will be produced towards the outer circumferential side from the location of contact between the base housing and the elastic wall, posing the risk of additional noise.
Furthermore, in the event that the air pressure acting on the working air chamber has changed from atmospheric pressure to negative pressure, if the elastic wall member is arranged to be brought into close contact with the base plate around its entire circumference, noise produced when the base plate and the elastic wall member strike each other will tend to become a problem.